If the home is complete I would go to the house and stay there untill the train passes. I would listen to it with the windows closed and also with the windows open. There are some buyers who would love to live with in walking distance to the train and some who will not. Keep in mind that the 3 most important words in real estate are Location, Location, Location
The best way to determine if it is noisy is to go to the site yourself, and listen. You are the only person who can make a determination as to what is acceptable to your needs. Obviously a home that borders the railroad is not as desirable as one that does not, so yes the location would be influence the price.